1. Field of the Invention
The present invention relates to video line interpolation, field rate converting and de-interlacing methods and apparatus for converting an interlaced standard video image into a progressive HDTV video image.
2. Description of the Prior Art
Up Converter is an interesting device for broadcasting at HDTV resolution or format an existing standard interlaced video signal into an HDTV video signal. An HDTV Up Converter can be essentially decomposed in three main parts: a field rate converter, a de-interlacer and a picture resizer. The combination of this three modules provide at the output a 60.00 Hz field (interlaced) or frame (progressive) rate having a 16:9 aspect ratio video signal. The major part in an HDTV Up Converter is the de-interlacer unit which converts the incoming interlaced video signal into a progressive video output. This progressive output permits in turn an easy vertical resizing. The present invention focuses on the first two modules of the HDTV up converter, thus providing improved methods and devices for the field rate conversion and for the de-interlacing a video image.
In today's broadcasting industry, more an more video presentations, such as movies and documentaries, must be converted from their original interlaced video format having 59.94 fields per second into the new High Definition Television (HDTV) video format, usually having 60.00 fields per second and a different image aspect rate, 16:9 rather than 4:3 for the standard format. For doing so, a large variety of solution have been proposed, each offering, however, only a partial solution to the problem.
For raising the field frequency of a video image from 59.94 Hz to 60.00 Hz, one new field must be added in each sequence of 1000 existing fields. This is usually done by repeating one video field, which does not give reliable results since the image will remain still for a longer time interval so it will often be perceived by the human eye. The second problem, i.e. de-interlacing a video image, is solved by inserting new video lines among the existing interlaced video lines.
In the patents U.S. Pat. No. 4,677,483 and GB-2,197,152A, fixed spatio-temporal filters are suggested for interpolation. However, this conversion technique introduces some artifacts such as line flicker and serration of diagonal edges. In U.S. Pat. No. 4,636,857, vertical filter and temporal filter are adaptively used according to a motion detector output. In U.S. Pat. No. 4,989,090, the adaptation is done by combining, in a suitable manner, vertical linear filter and temporal median filter results. The shortcomings are the resolution loss for moving parts in the picture, the staircase effect on moving diagonal edges. In U.S. Pat. No. 5,001,563, the interpolator is a spatio-temporal median filter in which the spatial inputs are selected along with estimated edge direction. There are still some defaults with this technique: the given edge detectors are not very reliable for a noisy or high frequency signal. This phenomenon produces some temporal flickers since the adaptation at a given edge can be different from frame to frame. The suggested spatial interpolation yields some resolution loss in the still parts of the picture. The median filter, which is excellent for preserving the edges, can give some unnatural results. In U.S. Pat. No. 5,019,903 and U.S. Pat. No. 5,347,599 the interpolations are edge-based and purely spatial which result in similar defaults caused essentially by the edge detection technique. Moreover, when the number of considered edge direction is small, for example three (3) directions for 45.degree., 90.degree. and 135.degree., the staircase effect becomes noticeable for nearly horizontal and long edge in the picture.
The present de-interlacer structure is similar to the one disclosed in U.S. patent application No. 08/916,960 now U.S. Pat. No. 5,936,676 and corresponding CAN patent application # 2,213,606 by the same applicants. However, nearly horizontal edges and new edge direction detection techniques will now be considered.
In the case of field rate converter from 59.94 to 60.00 Hz, the reference literature is very restrictive. Usually frame-repeating technique is used at the end of 1000 incoming fields or when a low motion or a still picture is detected in this time window. Generally, frame repeating can create a noticeable motion discontinuity.